The present invention relates to metals, in particular niobium, and products made from niobium metals as well as methods of making and processing the niobium metals.
In industry, there has always been a desire to form higher purity metals for a variety of reasons. With respect to niobium, higher purity metals are especially desirable due to niobium's use as a barrier material for superconductors, as a sputtering source for optical coatings, and for its potential use in electrical components such as capacitors. Furthermore, there is growing interest in high purity niobium-tantalum alloys for use in superconductor filaments, anti-reflective coatings, and possibly as a barrier film for copper interconnects in integrated circuits. Thus, impurities in the metal can have an undesirable effect on the properties and/or the performance of the articles formed from the niobium.
Niobium is found in nature almost exclusively in the form of an oxide (Nb2O5) as columbite ore or as a by-product of the benefaction of tantalum ores. The process used to produce high purity niobium metal typically consists of a chemical extraction and crystallization operation to yield a niobium-bearing ionic salt, a reduction of the ionic salt to produce a niobium metal consolidate, and vacuum melting of the consolidate to produce a high-purity niobium ingot. The vacuum-cast niobium ingot can then be mechanically worked into a variety of mill forms such as sheet, strip, bar, rod, and wire. The high purity niobium ingot and mill forms can also be hydrided, crushed, dehydrided, and subsequently processed to produce niobium powder suitable for capacitor or other powder metallurgy products.
The impurity elements in niobium can be classified as refractory metal impurities, other metallic impurities, and interstitial impurities. Refractory metal impurities include elements such as tantalum, molybdenum, and tungsten; these elements are infinitely soluble in, and have a similar vapor pressure as niobium and cannot be readily removed through vacuum melting. Therefore, refractory metal impurities must be removed prior to or during the niobium consolidation step using methods such as liquid-liquid extraction, fused salt electrolysis, and chemical vapor deposition (CVD) from niobium halides. Other metallic and interstitial impurities can be removed by vacuum melting of niobium. Metallic elements such as alkali, transition, and rare earth metals have higher vapor pressures and are volatized by vacuum melting processes such as Electron Beam (EB) melting, Vacuum Arc Melting (VAM), Vacuum Arc Remelting (VAR), or Electron Beam Float Zone Melting (EBFZM).
Interstitial impurities (nitrogen, oxygen, carbon, and hydrogen) are also removed by vacuum melting, or by annealing the niobium metal at a high annealing temperature in a strong vacuum. Hydrogen typically is readily outgassed from nitrogen at temperatures above about 400° C. in vacuum. Dissolved nitrogen (N) is removed from niobium as diatomic nitrogen (N2); the kinetics of the reaction is a function of the equilibrium concentration of nitrogen in the niobium as determined by Sievert's law, (E. Fromm and G. Hörz, Intern. Met. Rev., 25 (1980), pp. 269-311, incorporated in its entirety by reference herein).       c    N    =                    P                  N          2                      ⁢          exp      ⁢              (                  Δ          ⁢                                           ⁢                                    S              N              0                        /            R                          )              ⁢          exp      ⁢              (                              -            Δ                    ⁢                                           ⁢                                    H              N              0                        /            RT                          )            where       Δ    ⁢                   ⁢          S      N      0        =                    -        70.3            ⁢                        J          /          mol                ·        K            ⁢                           ⁢      and      ⁢                           ⁢      Δ      ⁢                           ⁢              H        N        0              =                  -        178            ⁢              kJ        /        mol            Niobium can be decarburized with oxygen either dissolved in the metal or from the atmosphere to form carbon monoxide. The equilibrium concentration of carbon can be estimated for in-situ or atmospheric decarburization as follows, (E. Fromm and H. Jehn, Met. Trans., 3 (1972), pp. 1685-1692 and E. Fromm and H. Jehn, Z. Metallkd., 58 (1968), pp. 65-68, incorporated in their entireties by reference herein).For in-situ decarburization       x          [      C      ]        =                    p        CO                    p                  O          2                      ⁢          exp      ⁢              (                                            -              5600                        /            RT                    -          11.2                )            For atmospheric decarburization       x          [      C      ]        =                    p        CO                    p                  O          2                          1          /          2                      ⁢          exp      ⁢              (                                            -              12750                        /            RT                    -          5.30                )            Typically, carbon is reduced in niobium by assuring that there is a stoichiometric amount or excess of oxygen to completely convert carbon to carbon monoxide. Any remaining oxygen is removed in the form of niobium suboxides by heating at near or above the melting point of niobium within a very high vacuum.
In using a combination of the above-mentioned processes, the ability to produce niobium metal having a purity of 99.999% and better has been demonstrated, (A. Koethe and J. I. Moench, Mat. Trans., JIM, 41 No. 1 (2000), pp. 7-16, incorporated in its entirety by reference herein). These or similar methodologies also have been utilized to manufacture tantalum metal having 99.999% purity, (International Application No. WO 87/07650 and European Patent Application No. EP 0 902 102 A1, incorporated in their entireties by reference herein). The ability to produce both high purity niobium and tantalum lends to the creation of techniques for the manufacture of high purity niobium-tantalum alloys.
However, chemical purity is not the lone parameter critical to the functionality and performance of high purity Nb or Nb—Ta alloys. There is a desire to have a high purity Nb or Nb—Ta product having higher purity, a fine grain size, and/or a uniform texture. Qualities such as fine grain size and homogeneous texture can be an important property for superconductor barrier sheets where formability is paramount and for sputtering targets where a uniform microstructure void of (001) localized texture bands can lead to improved uniformity of thickness of the sputtered deposited film. Further, other products containing the niobium having fine grain size can lead to improved homogeneity of deformation and enhancement of deep drawability and stretchability which are beneficial in making capacitors cans, laboratory crucibles, and increasing the lethality of explosively formed penetrators (EFP's). Uniform texture in tantalum containing products can increase sputtering efficiency (e.g., greater sputter rate) and can increase normal anisotropy (e.g., increased deep drawability), in preform products. (C. A. Michaluk, D. B. Smathers, and D. P. Field, Proc. 12th Int. Conf. Texture of Materials, J. A. Szpunar (ed.), National Research Council of Canada (1999) pp. 1357-1362, incorporated in its entirety by reference herein).